Actuator assemblies including a piston-like ram member may use pressurized fluid to displace the ram member to move a closure member of a control valve between an open position and a closed position. Specifically, the ram member may have a piston head that is disposed within an interior of a cylinder, and fluid under pressure is provided to the cylinder interior to act on the piston head to displace the ram member. An end of the ram member may extend out of the cylinder and abut an end of a rod member, and the opposite end of the rod member may be directly or indirectly coupled to the closure member of the control valve. Accordingly, movement of the ram member by fluid pressure displaces the closure member of the control valve.
Often, fluid under pressure is provided in the cylinder interior to maintain the closure member of the control valve in an open position. The fluid under pressure may be provided by a manual hand pump or an electrical pump. Due to leakage in the system that brings the pressure to the threshold pressure, the manual pump may need to be periodically actuated to maintain fluid pressure in the system to a level that maintains the valve member in the open position. Alternatively, the electrical pump cycles on to increase pressure in the cylinder volume to maintain the valve member in the open position.
During the summer or when the system is subjected to relatively high temperatures, fluid in the system may subject to thermal expansion. This thermal expansion causes pressure in the system to increase, and the pressure increase acts on the ram member to provide a force that is greater than that necessary to maintain the closure member of the control valve in the open position. This excess force may damage a mechanical stop that prevents the rod member from displacing past a point corresponding to the fully open position to prevent damage to the closure member and/or the valve sear of the control valve. Accordingly, a relief valve may be triggered when the pressure increases past a maximum cracking pressure. However, after the temperature lowers, system pressure also lowers and the system pressure may be lower than the pressure necessary to maintain the closure member of the control valve in an open position. To increase system pressure, the manual pump must be actuated or the electrical pump must be cycled on. However, because the actuator assembly and the control valve may be at a remote location, it is expensive and time consuming to have a technician travel to the valve to operate the manual pump. Even with the electrical pump, the cycling on and off as the system fluid pressure increases and decreases is undesirable.
Conventional methods used to compensate for the thermal expansion of the system fluid typically utilize a bladder accumulator device plumbed into an operating port of the actuator assembly. The bladder accumulator device requires a pre-charged nitrogen head pressure on the topside of the bladder with a small volume of system fluid used by the actuator assembly. As the temperature increases and the system fluid expands, the bladder will compress the nitrogen and compensate for the thermal expansion of the system fluid to prevent system pressure from reaching the cracking pressure of the relief valve. Although this configuration is simple in concept, it is difficult to maintain the nitrogen gas head pressure in the topside of the bladder. Nitrogen gas at high pressures 2000 to 3000 psig will eventually leak out of the containment vessel of the bladder accumulator device. The small nitrogen molecule will leak by both conventional pipe thread fittings and straight thread O-ring sealed fittings. This slow gas egress will require the customer to periodically check and maintain the proper gas head pressure to ensure the function of the thermal compensation system.
Accordingly, there is a need to eliminate the use of the conventional bladder accumulator device while still preventing the system pressure from reaching the cracking pressure of the relief valve and while eliminating excessive pressure acting on the rod member that is coupled to the closure member of the control valve.